Mixed gas containing multiple component gases to be supplied into gas equipment such as a semiconductor manufacturing apparatus chamber is required to keep the total flow rate thereof and the mixing ratio between or among the component gases at predetermined values, respectively.
In order to obtain such mixed gas, a flow rate ratio control apparatus in Patent Literature 1 separately controls the flow rates of the component gases by, in a flow path mechanism including: supply lines through which the component gases separately flow; and a mixed gas line that is connected with the downstream end parts of the supply lines with the downstream end parts merging together, providing flow rate control devices in the supply lines, respectively.
Target flow rates to be respectively set in the flow rate control devices are calculated from the values of the target total flow rate and the target mixing ratio that are kept temporally constant. For this reason, since the target flow rates of the component gases also take constant values with respect to time, the flow rate control devices always stabilizes the actual flow rates of the component gases at the target flow rates, respectively and correspondingly, and thereby the target total flow rate and target mixing ratio of the mixed gas can be achieved.
Meanwhile, in such a flow rate ratio control apparatus, it is sometimes desired to change the target total flow rate or target mixing ratio of the mixed gas from some value to another value. In such a case, the target total flow rate or target mixing ratio to be inputted to the flow rate ratio control apparatus is discontinuously changed and then inputted, and therefore the target flow rates to be set in the flow rate control device are also instantaneously changed to values corresponding to the changed target total flow rate or target mixing ratio, respectively.
However, a response speed to an actual flow rate of each component gas with respect to a target flow rate set in a flow rate control device is limited, and therefore if there is a change period during which the target flow rate changes stepwise from some target flow rate value to another target flow rate value, the actual flow rate does not follow the change in the target flow rate. Also, there are the difference in characteristics of a flowing component gas between or among the flow rate control devices, and an instrumental error depending on a flow rate control device, and therefore the deviation amount of the actual flow rate from the target flow rate in the change period may increase. Depending on a change condition of the target total flow rate or the target mixing ratio, a target flow rate of some component gas increases, whereas a target flow rate of another component gas decreases, and therefore between a rise step response and a fall step response, the large difference in characteristics easily occurs.
Accordingly, when the target total flow rate or the target mixing ratio is changed, in a predetermined time period after the time point of the change, during which the flow rates of the component gases vary, the total flow rate and mixing ratio of the mixed gas are not stable, and unless the flow rates of the component gases are stabilized respectively at constant values after some time has passed, desired mixed gas cannot be obtained.